If you direct solder, you might want to make sure your motor spins in the right direction. If it spins the wrong way, swap two of the wires. Once you have it right, then you can solder them permanently and heatshrink it.
Oh, and instead of using the 4mm connectors as a direct solder solution, you might want to try
these instead. Just cut the plastic jacket off the inside connector and solder the wires in each end (don't crimp). They're cheaper and fit the wire better so there is less solder needed. Either way works though.